Orientational order parameters in the nematic and smectic A a binary mixture of smectogenic liquid crystals

High resolution optical transmission data of a binary mixture of two smectogenic compounds of which one having NCS terminal group exhibiting smectic A phase and the other with CN terminal group showing Smectic Ad phase have been analyzed to determine the orientational order parameters as a function of temperature. The orientational order parameters (OOPs) value increases with decreases of temperature both the nematic and smectic A phases. Haller’s extrapolation procedure has been used to determine the orientational order parameter <P2> from the birefringence data. The temperature dependence of birefringence were measured from the transmitted intensity data for wavelength λ=632.8 nm and λ=532 nm. The modified four parameter model consistent with the mean-field theory for weakly first order transition have been used. This result has also been successfully fitted with those calculated from the Maier-Saupe theory for the nematic and McMillan’s theory for the Sm A phase.